1. Field of the Invention
The present invention relates to sensors for the detection of ammonia.
2. Description of Related Art
Continuous monitoring of analytes can be accomplished by a number of methods, including those based on electrochemical, calorimetric and fluorometric detection. Ammonia sensors utilizing electrochemical methods take advantage of the fact that at physiologic pH, ammonia is protonated to form ammonium ions, which increase the electrical conductivity of the solution proximal to the electrode.
Colorimetric monitoring is achieved by the detection of color or loss of color using either qualitative visual observation or spectrophotometric measurement of the color intensity. Such colorimetric monitoring has an important limitation: sensitivity of detection. To improve sensitivity, fluorescence-based sensors can be used. However, not all colorimetric assays may have fluorescent equivalents.
Ammonia can be monitored using an optical sensor. The detection of analytes by optical sensors requires the development of fluorescent transducers which are specific for different analytes, coupled to a media or surface capable of transmitting light, such as optical fibers. Optical transducers for the detection of ammonia can be modulated by ammonium or ammonia.
Detection of ammonium requires an ammonium specific ionophore coupled to a chromophore that changes its absorption spectrum upon protonation, and a lipophilic anionic site. As such, sensors based on the detection of ammonium can be expensive and complex.
Detection of ammonia requires a protonated pH sensitive indicator (IND.sup.+) which changes its absorption or fluorescence spectrum upon deprotonation: EQU INDH.sup.+ +NH.sub.3 .fwdarw.IND+NH.sub.4.sup.+
There is also a drawback to designing a sensor based on detection of ammonia: namely the rapid protonation of ammonia at physiologic pH. The pK.sub.a of ammonium is 9.3, which is not a pH that supports maximum enzyme activity.
A fiber-optic fluorescing sensor to detect ammonia was reported by Wolfbeis and Posch (Anal. Chim. Acta, 185: 321-327 (1986)). The sensing material was prepared as a buffered emulsion of dyes in an aqueous solution entrapped in a silicone rubber. The ammonia response for this sensor was not always reversible, and the signal intensity was low as the dye was dissolved in aqueous buffers before being prepared as an emulsion in the polymer.
Other methods of detecting ammonia have been to sequester an indicator solution from the sample (Rhines, T. D.; Arnold, M. A., Anal. Chim. Acta, 227: 387-396 (1989)) or to use an anionic colorimetric pH indicator paired with a quaternary ammonium cation, which has been solubilized in a silicone elastomer (Werner, T.; Klimant, I.; Wolfbeis, O. S. Analyst, 120: 1627-1631 (1995)).
Hydrophobic polymers, optically transparent and permeable to the analyte of interest, are used with optical sensors when the analyte is a vapor or gas and is capable of diffusion into a hydrophobic membrane. A complication arises when hydrophobic polymers are used with certain fluorescent dyes. Sensors for ammonia require a protonated indicator. When combined with a hydrophobic membrane for the detection of ammonia, polyanionic pH indicators, which are the common variety of protonated indicator and the type used in the fluorescent urea sensor described in Rhines and Arnold (Anal. Chim. Acta, 231: 231-235 (1990)), do not produce an activated and protonated fluorophore.
While various indicators for ammonia are known, many ammonia sensors exhibit problems with interferences from pH and CO.sub.2 effects, low sensitivity, slow response times and reversibility. Additionally, polyanionic pH indicators typically used in the detection of ammonia have not been successfully used in conjunction with hydrophobic polymers. From a manufacturing standpoint, it would therefore be desirable to develop an inexpensive ammonia sensor that has a high sensitivity, fast response time, and is reversible. It would also be advantageous for the sensor to be able to function in conjunction with sensors detecting other analytes.